1. Field of the Invention
The present invention relates to an ophthalmologic apparatus.
2. Description of the Related Art
In a conventional ophthalmologic apparatus such as a fundus camera, a light source such as a xenon tube is used as a light source for imaging a fundus of a target eye to be examined. Since the light source generally degrades with time, the imaging light amount varies even when the light emission of the light source is always stopped at the same timing.
Therefore, in order to control the imaging light amount, the light reflected from the target eye is monitored by a light receiving element such as a photo diode, and the output from the light receiving element is integrated by an integration circuit equipped with an operational amplifier and a capacitor. Then, there is a known technique of controlling the imaging light amount in a manner such that the light emission is stopped when the output voltage of the integration circuit becomes larger than a reference voltage based on the comparison between the output of the integration circuit and the reference voltage. Further, there is a known technique of changing the imaging light amount according to the sensitivity of a film used for an imaging operation, or the interest and preference of an examiner as to a portion to be imaged, by changing the reference voltage (see Japanese Patent Application Laid-Open No. 60-190930).
However, as an adjustment range of the imaging light amount becomes wider, a difference in light amount between the maximum light amount and the minimum light amount in the adjustment range of the imaging light amount increases. In addition, since the output voltage of the integration circuit decreases at it approaches the minimum light amount, the reference voltage which is compared with the output voltage of the integration circuit also decreases.
Further, a change in the output voltage of the integration circuit and the reference voltage in the vicinity of the minimum light amount is smaller, when the imaging light amount changes (for example, when the imaging light amount becomes a half), compared to a change in the output voltage of the integration circuit and the reference voltage in the vicinity of the maximum light amount when the imaging light amount changes (for example, when the imaging light amount becomes a half). More specifically, the resolutions of the output voltage of the integration circuit and the reference voltage are degraded as it approaches the minimum light amount. Further, when the voltage is low, the output voltage may be easily affected by noise.
Thus, the output voltage of the integration circuit and the value of the reference voltage become different from the desired values, so that the precision in the light amount control is degraded.